1. Field of the Invention
A method of manufacturing a matrix for producing optical disks without the medium of a master.
2. Background in the Art
The invention relates to a method of manufacturing a matrix for producing optical disks, whereby a photoresist film is applied to an unstructured matrix plate, and the photoresist film applied is structured, and whereby said structuring of the photoresist film applied successively comprises the steps of selectively exposing and developing the photoresist film applied.
A method of this kind is known from the publication: G. Bouwhuis, editor, "Principles of optical disk systems", Adam Hilger Ltd., Bristol 1985, which describes the conventional method of mass-producing optical disks, also called replicas. The method makes use of an injection moulding process and matrices which are manufactured through the medium of a master; that is, first a master is made and then a matrix by means of the master.
Optical disks contain information recorded in a geometric structure. More in particular this geometric structure consists of pits and lands, said pits being provided in concentric circles or in a spiral in a surface of a respective optical disk. Also grooves are sometimes used to form the structure of optical disks. Generally this. concerns media capable of being written on, such as CD-Recordable and magneto-optical disks. The known method of mass-producing optical disks of this type can be subdivided into three stages.
During the first stage information stored on an information carrier, in particular a magnetic tape, is converted into a sequence of electrical pulses. These pulses control the intensity of an exposing unit, in which a substrate plate, for example of glass, provided with a photoresist rotates. Thus small areas in the photoresist are exposed, which are dissolved in an aqueous basic solution in a next step.
The result is a structure of pits, as is also desired for the eventual optical disk. This intermediate product is called a master.
During the second stage one surface of the master is plated with a thin metal film. This metal film serves as an electrode for galvanically growing a relatively thick metal film, in particular nickel.
After physically separating the substrate plate and the thick metal film the information structure mentioned above is present on a surface of the thick metal film in inverted position. Then the thick metal film or sheet or shell is cleared of any residual photoresist, cut to size and used as a so-called matrix during the third stage yet be described hereafter. The steps carried out during the first and second stages may also be executed in a fully automated system, such as the applicant's AMS 100.
During the third and final stage the matrix is placed in the mould of an injection moulding machine, by means of which large series of optical disks, such as audio and/or video disks are produced. Finally the optical disks are metallized and provided with a protective coating and a label or a print. The third stage is a fully automatic production process.
The above-described known method is used worldwide, whereby use is made of a positively acting photoresist, whose main characteristic is the fact that the exposed areas of an exposed film of said positively acting photoresist are removed by developing, while those areas that have not been exposed remain intact.
The drawbacks of said galvanic growing process during the second stage discussed above are that a great deal of manual operations have to be performed and that a relatively great amount of chemicals must be used. Although said operations are carried out by a handling unit in an automated process, the required very high complexity of this unit may lead to high costs and reliability problems. The electroplating time cycle is typically about 1 hour, which is long.
Although it is possible, by a so-called "family making" process, to make more than one matrix per master, by using a matrix itself as an information carrier and making use of a galvanic process, this method is being applied less and less in the industry, precisely because of the above-mentioned drawbacks. Also smaller batches of disks, such as for CD-ROM disks, have contributed towards ending the practice of making a "family".